The present invention relates to a method and a device for accurate control and measurement of small forces and small movements in a loading device intended for essentially greater forces and movements.
Investigation of the mechanical properties of metals or other materials demands careful measurements of the load and of the deformation of a test piece caused by said load, i.e. a simultaneous measurement of load and movement.
Materials testing laboratories have normally access to testing machines with a load capacity from 50 kN and upwards. In tests demanding considerably smaller loads, about 1000N or less, the prescribed accuracy in determining the load can often not be attained.
A frequently used method of solving said problem is to insert an additional force transducer with suitable measuring range in series with the test piece. At tension loads this solution can be possible because the transducer can be protected against harmful overloads occurring for instance when the test piece breaks. On the other hand, at compressive loads it is difficult to protect the force transducer effectively against such harmful overloads. This depends upon the fact that it is difficult to operate the (often servo controlled) machine with sufficient accuracy. It may also result in that the test piece is exposed to harmful overloads and may break already when it is affixed in the machine.
For test pieces having very low strength, devices for measuring the deformation (extensometers) can give rise to forces against the test pieces which influence the fracture behaviour and the measured forces in a non-acceptable way.
The purpose of the present invention is to provide a method which eliminates the above mentioned disadvantages when for example a testing machine is used for determination of the bending, compressive or tensile strengths or of the fracture toughness, within a loading range for which the machine is not intended to be used.
The principle is based upon the physical fact that, in a suitably designed annular body, yoke, loaded in two diametrically opposite points, a well-defined usually linear relationship between force and deformation, D, is obtained within the elastic range, D being measured as the displacement of said points in relation to each other in the direction of the force.